1. Field of the Invention
The present invention relates to an organic light-emitting device (OLED), and, more particularly, to an organic light-emitting device having a doped hole transporting layer (HTL) and/or a hole injecting layer (HIL).
2. Description of the Related Art
An organic light-emitting device, such as a self-emissive display, has an ideal structure in that it is compact and light-weight and has simple components and processes. The OLED also has several other advantages, such as the capability to secure high definition and a wide viewing angle, the capability to realize a superb moving picture and a high contrast, and an electrical characteristic suitable for a mobile device due to a driving capability requiring low power consumption and low voltage.
As for the general structure of the organic light-emitting device, an anode is located on a substrate. An organic layer, including an emission layer (EML), is located on the anode. A cathode is located on the organic layer. The organic layer may further include a hole injecting layer (HIL) and/or a hole transporting layer (HTL) between the anode and the emission layer, as well as an electron transporting layer (ETL) and/or an electron injecting layer (EIL) between the emission layer and the cathode.
The organic light-emitting device has a driving principle as follows. When a voltage is applied between the anode and the cathode, holes are injected from the anode through the hole injecting layer and the hole transporting layer into the emission layer, while electrons are injected from the cathode through the electron injecting layer and the electron transporting layer into the emission layer. The holes and electrons, which are injected into the emission layer, are recombined at the emission layer, thus generating excitons. While being transited from an excited state to a ground state, these excitons emit light.
Generally, hole mobility in the organic layer is about 10 to 100 times as high as electron mobility. This has an influence on enhancement of luminous efficiency and improvement of lifetime of the device, both of which are needed to commercialize the organic light-emitting device.
In order to obtain a high luminous efficiency, equilibrium must be maintained between a quantity of the electrons injected into the emission layer and a quantity of the holes injected into the emission layer. However, as mentioned above, the hole mobility is higher than the electron mobility, and therefore the quantity of the holes injected into the emission layer becomes more than that of the electrons. In order to alleviate this problem, a hole blocking layer is additionally interposed between the emission layer and the electron transporting layer to confine the holes within the emission layer. Thereby, the luminous efficiency is enhanced.
Meanwhile, because work functions of each layer constituting the organic layer are different, charges are accumulated at each interface between the layers. Due to this charge accumulation, the organic light-emitting device becomes deteriorated. Thus, the lifetime of the organic light-emitting device becomes shortened. However, as described above, because the hole mobility is higher than the electron mobility, the holes accumulated at the interface between the emission layer and the hole injecting layer or the hole transporting layer and the anode are more than the electrons accumulated at the interface between the cathode and the electron injecting layer or the electron transporting layer and the emission layer.
To improve the situation created by this charge accumulation, according to U.S. Pat. No. 5,601,903, an emission layer or a charge transporting layer adjacent to a hole injecting electrode is doped with a material which has a conduction band lower than that of the emission layer or the charge transporting layer, and the emission layer or the charge transporting layer adjacent to an electron injecting electrode is doped with another material which has a valence band lower than that of the emission layer or charge transporting layer. Thereby, an energy barrier between the emission layer and the charge transporting layer is lowered to enhance the charge mobility, so that it is possible to lessen the problem of the charge accumulation. However, this facilitates the outflow of charges from the emission layer, so that it is possible to adversely influence the luminous efficiency.